Dietary therapy for long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) includes avoidance of fasting and a low long-chain fatty acid (LCFA) diet supplemented with medium chain triglyceride (MCT). Acute episodes of hypoglycemia, acidosis, and cardiomyopathy are largely prevented with this therapy, but chronic complications, such as exercise-induced rhabdomyolysis, are not prevented. MCT may provide an alternate fuel source during exercise. Forty percent of a currently followed cohort is overweight or obese and most subjects are lipogenic at rest. The first objective of this study is to determine if a high protein, reduce carbohydrate diet will decrease anabolism, decrease lipogenesis at rest, and improve metabolic control in children with LCHADD. The second objective is to determine if MCT supplementation prior to exercise decreases the risk of rhabdomyolysis of subjects with LCHADD. The metabolic response to a high protein, reduced carbohydrate diet compared to a standard LCHADD diet and exercise tolerance with and without supplemental MCT prior to exercise testing will be assessed in a randomized crossover design.